In a recent report, Hallgren et al. (J. Biol. Chem. 252: 1014-1022, 1023-1033, 1034-1040, 1977) described the isolation and chemical structure of several oligosaccharides in urine from 3 pregnant or lactating women. Some of these were oligosaccharides with human Lea or Leb blood group activity. Using a radioimmunoassay developed in our laboratory we examined the urinary excretion of Leb-active oligosaccharides in urine of 138 pregnant or lactating women of different Lewis blood types. The amount of Leb-active oligosaccharides in urine during pregnancy and lactation correlates with Lewis blood type and with the strength of expression of Leb antigen on erythrocytes. The site of origin of these oligosaccharides has not been defined but several lines of indirect evidence suggest they may originate in mammary gland.